Several industrial processes produce streams containing one or more esters of carboxylic acids, RO.CO.R′, where R and R′ are selected from among hydrocarbyl radicals or substituted hydrocarbyl radicals. Examples of such industrial processes include those for production of vinyl alcohol polymers or terephthalic acid. Carboxylic acid esters can be hydrolyzed to generate the corresponding acid and alcohol, as shown in Equation 1. Equation 1 is an equilibrium reaction and requires an excess of water to drive the reaction well to the right hand side.RO.CO.R′+H2OROH+HO.CO.R″  [1]
Processes for production of polyvinyl alcohol (PVA) and its derivatives are described by Marten in “Vinyl Alcohol Polymers” in Kirk-Othmer Encyclopedia of Technology, John Wiley & Sons, Inc. A variety of vinyl ester monomers can be polymerized to form a polymer, of which polyvinyl acetate (PVAc) is preferred. PVAc then is further reacted to manufacture PVA. Commonly, PVAc is reacted with methanol (MeOH) to form PVA and methyl acetate (MeOAc). The components of the polymerization reaction mixture are continuously separated. Unreacted monomer can be stripped from the reactor using, for example, methanol vapor. The overhead fraction from the stripper comprises a mixture of vinyl ester monomer and at least one solvent such as methanol. The vinyl ester monomer is then extracted for recycle to the polymerization reactor. In the production of polyvinyl alcohol (PVA) MeOAc is produced as a by-product at a ratio of 1.68 tons of MeOAc per ton of PVA.
One outlet stream typically comprises a mixture including MeOAc, MeOH and a small amount of water. The weight ratio of these components varies over a range of relative concentrations, among which a typical composition is approximately 75% MeOAc, 23% MeOH and 2% water. Among these components MeOH and water have relatively low value when compared to the values of MeOAc and acetic acid (HOAc).
The MeOH and MeOAc can be distilled off and, at the same time, water can be added in order to obtain an aqueous PVA solution. However, there are disadvantages to this approach. The resulting PVA suspension is fine, difficult to filter, and so the process is uneconomical. Further, this approach requires time-consuming, energy intensive and hence expensive distillation of large amounts of solvents requiring a plurality of distillation columns. Several approaches have been undertaken to improve the chemical efficiency and economics of processes for production of PVA. In particular, efforts have been directed to recycle of solvents and processing of the outlet streams to recover valuable by-products.
Kowaka et al. in U.S. Pat. No. 6,743,859 issued in 2004 describe a method for production of high-strength PVA with a high degree of saponification. The apparatus for the process of '859 includes an outlet line for recovery of MeOH and MeOAc identified in FIGS. 1 through 3 by the reference numeral 7, however no details are presented for the process for the separation and recovery of those components.
Bauer et al. in U.S. Pat. No. 6,576,720 issued in 2003 describe an alternative approach in which a liquid phase comprising MeOH, MeOAc and HOAc is recycled for use in further polyvinyl ester transesterification. The process of '720 can be used for other alcohols and esters. The mixture of alcohol and corresponding ester is recycled for use as the polymerization reaction medium. Make-up comprising one or both of MeOH and HOAc is provided to maintain the composition of the recycle mixture. The PVA is produced and isolated using saponification with KOH and then neutralization, preferably with a strong acid such as HCl. Thus water, less than 1% by weight in the initial reaction mixture, is produced by both the reaction of MeOH and HOAc and the neutralization process.
MeOAc may be sold or further hydrolysed to recover HOAc.
Kim et al. in U.S. Pat. No. 5,770,770 issued in 1998 describe a reactive distillation process for the well known equilibrium reaction 2 for the recovery of MeOH and HOAc from catalytic hydrolysis of MeOAc. Reaction 2 is a specific example of the type of reaction shown in Equation 1.MeOAc+H2OMeOH+HOAc  [2]
It has long been recognized that this reaction could be used to recover HOAc from MeOAc from a PVA manufacturing process as described, by example, by Adelman et al. in U.S. Pat. No. 4,352,940 issued in 1982. It also was recognized in '940 that it was necessary to minimize the amount of water used in the process to reduce the costs of recovery and re-use of the products from the reaction in the PVA manufacture process. However, when a minimum amount of water is used, the equilibrium reaction 2 lies to the left hand side. Reaction 2 can be driven to the right hand side only by continuous removal of at least one of the products.
In an alternative approach for treatment of the PVA manufacture outlet stream, MeOAc can be separated from the mixture using extractive distillation. One example of this method is described by Xiao et al. in Chemical Engineering Science, volume 56, pages 6553-6562 (2001). In the first column, water is added to the liquid stream from the PVA plant. The volatiles from the first column are then hydrolyzed in a fixed bed reactor containing a bed of an acidic catalyst which catalyzes the hydrolysis of MeOAc to MeOH and HOAc. The effluent stream from the fixed bed reactor is distilled in a second distillation column to provide a volatiles stream and a bottoms stream. The volatiles from the second column are recycled for mixing with further MeOAc feed to the first extractive distillation column. The bottoms from the second column are separated into a water rich stream and a HOAc rich stream by distillation in a third column. The bottoms from the first column are separated by distillation in a fourth column into a water rich stream and a MeOH rich stream. Thus the overall process for recovery of HOAc requires four distillation columns and a fixed bed reactor. Further, to drive well to the right hand side the well known catalytic MeOAc hydrolysis equilibrium reaction shown as Equation 2, it is necessary to use a large amount of water. Thus the process is energy intensive as that water must be volatilized in both the second and third columns.
Each of the above processes requires use of a plurality of columns and reactors to react, separate and recover the components of the stream from the PVA manufacturing reactor. Consequently, capital and operating costs are high. Further, when water is added, either as reagent or for extractive distillation, that water must also be separated, which is a costly and time consuming feature.
Hoyme et al. in U.S. Pat. No. 6,518,465 issued in 2003 describe another concept based process, derived from simulations using the commercial available program Aspen Plus, in which the stream containing MeOAc from PVA manufacture is reacted in a reactive distillation column to produce DME and HOAc. Water was added to hydrolyze MeOAc and thereby generate HOAc which is recovered. The molar ratio of water in the process stream is between 0.05% and 20%, and preferably is between 0.3% and 3%. In this process it is recognized that methanol also may react to generate dimethyl ether (DME) and water in the also well known acid catalyzed equilibrium reaction shown in Equation 3. The process of Hoyme et al. in '465 is basically hydrolysis of MeOAc to HOAc by addition of water.2MeOHMe2O+H2O  [3]